A number of devices for the detection of a given analyte in a biological sample are available in a variety of design formats. In addition to devices requiring a multiplicity of tubes, vials and support structures, immunoassay devices are encased in housings or casings of varying designs, shapes and colors. One-step assays are those assays in which the user adds a biological sample to a sample application port and receives a positive or negative signal corresponding to the presence or absence of a test analyte in the biological sample. These assays are complex devices that must incorporate into the assay housing a number of immunochromatographic elements. Since the one-step assay has a limited means for sample manipulation, design variations that reduce the number of false positives and improve the sensitivity of the assays are a significant improvement to the art.
There are a number of one-step immunochromatographic assay devices available in the art. These systems all generally permit a sample to be introduced into a receiving port. The sample is placed in contact with a particular reagent which is most often linked to a chromogenic tag. Detectable molecules in the sample complex with reagent and are wicked through the assay where they are ultimately immobilized, usually by a second reagent specific for the test analyte, in a position that generates a positive reaction signal. Additional signals included in the assay provide a negative reaction indicator and test complete indicator.
Immunochromatographic assays of this sort may have a number of potential problems. If the flow of liquid from the sample through the wicking material is too fast, the detectable molecules cannot keep up. They are left behind and are not accurately detected by the assay. The choice of assay materials and their dimensions determine the rate of liquid flow through the assay. The materials and dimensions also influence the evenness of the flow of the detecting molecules through the assay as well as the compatibility of various analytes testable with the sample. Both of these features influence the accuracy of the assay.
In addition, one-step assay results are often compromised by the presence of water marks on the wicking surface that result from liquid pooling beneath the surface. Excess liquid in a one step assay not only creates water marks that may be falsely interpreted as a positive signal, but the excess fluid can flood the one-step device and produce false-negative signals.